Why does a petrol-hybrid car with a turbo engine also have an electric supercharger? And why were electric superchargers not used in the past?
Traditional superchargers are belt-driven compressors that force air into the combustion chamber of a petrol- or diesel-fuelled engine. This type of forced induction achieves a boost in engine power similar to what a turbocharger does.
The main difference is that a belt-driven compressor starts its forced-induction function the moment the engine is running. There is no lag in power boost, whereas the turbocharger needs a few moments for the engine to build up sufficient exhaust gases to start spinning its compressor.
There are engines that have both a turbocharger and a supercharger. The best known one is Volkswagen’s twin-charge engine, where a belt-driven compressor provides the boost at lower engine speeds before switching over to the turbocharger.
Electric superchargers are meant to provide a boost immediately, filling in the gap before the turbo unit builds up the boost.
Mercedes-Benz uses this technology.
Instead of being mechanically connected to the engine, an electric compressor does the work of forcing air into the engine’s combustion chamber. This can be efficiently switched on or off instantly whenever the engine management system calls for a low-end boost.
In Mercedes-Benz vehicles, the compressor is powered by a 48-volt battery that is kept charged by the integrated starter-generator, which is part of the mild-hybrid system.
Even if it is just for a low-speed boost, the compressor needs a motor with at least a 48-volt supply if it is to force the necessary volume of compressed air into the engine. The standard 12-volt system is unable to deliver the power needed to drive an electric compressor in this way. This is also why such a supercharger has not existed in the pre-high voltage era.
A full electric compressor to replace the turbocharger would, of course, require a much larger motor and even higher voltage that current hybrid systems are not built for.
